Obsessive-compulsive disorder (OCD) is a severe and heterogeneous disorder with a considerable rate of treatment nonresponders. It is important to understand the neural underpinnings of the disorder so that they can inform and improve treatments. A well-established target candidate for the underlying psychopathology in OCD is the error-related negativity (ERN) as measured in performance monitoring ( 1 Endrass T. Ullsperger M. Specificity of performance monitoring changes in obsessive-compulsive disorder. Neurosci Biobehav Rev. 2014; 46: 124-138 Crossref PubMed Scopus (99) Google Scholar ). The ERN is important for the adaptation of behavior (e.g., affect/motivation) and subsequent actions. Many studies have observed an increased ERN and discussed this phenomenon as a potential biomarker or endophenotype for OCD. However, despite the need to identify qualified endophenotypes early in the course of the disorder—or even before a clinical threshold is reached, assuming it might be easier to treat at this early stage—evidence from pediatric samples and early-onset OCD is scarce. The study by Suzuki et al. ( 2 Suzuki T. Gu P. Grove T.B. Hammond T. Collins K.M. Pamidighantam P. et al. Abnormally enhanced midfrontal theta activity during response monitoring in youths with obsessive-compulsive disorder. Biol Psychiatry. 2023; 93: 1031-1040 Abstract Full Text Full Text PDF Google Scholar ) in the current issue of Biological Psychiatry provides evidence for an increased ERN and theta band power in a group of children and adolescents with early-onset OCD compared with an unaffected control group. Consistent with previous studies, the authors observed increased amplitudes and theta power in OCD not only following errors but also following correct responses. In addition, they analyzed intertrial phase coherence in the theta range—a measure for the synchronization of the phase of the signal between trials independent of amplitude ( 3 McLoughlin G. Gyurkovics M. Palmer J. Makeig S. Midfrontal theta activity in psychiatric illness: An index of cognitive vulnerabilities across disorders. Biol Psychiatry. 2022; 91: 173-182 Abstract Full Text Full Text PDF PubMed Scopus (9) Google Scholar )—but group effects were not observed. Enhanced ERN and theta power have been suggested as biomarkers or endophenotypes for OCD. Although there is substantial evidence supporting the notion of performance monitoring abnormalities in OCD, we will discuss the following aspects: 1) the function of ERN and theta power for adaptive behavior and in mental disorders; 2) performance monitoring as a specific versus transdiagnostic biomarker; and 3) functional role and sensitivity to interventions. SEE CORRESPONDING ARTICLE ON PAGE 1031 SEE CORRESPONDING ARTICLE ON PAGE 1031 Abnormally Enhanced Midfrontal Theta Activity During Response Monitoring in Youths With Obsessive-Compulsive DisorderBiological PsychiatryVol. 93Issue 11PreviewResponse monitoring, as reflected in electroencephalogram recordings after commission of errors, has been consistently shown to be abnormally enhanced in individuals with obsessive-compulsive disorder (OCD). This has traditionally been quantified as error-related negativity (ERN) and may reflect abnormal neurophysiological mechanisms underlying OCD. However, the ERN reflects the increase in phase-locked activities, particularly in the theta-band (4–8 Hz), and does not reflect non–phase-locked activities. Full-Text PDF